A wide range of signals and related protocols exist for the use of radio frequency signals in communication systems and other devices, such as radar systems. In some applications, it is desirable to determine or confirm the existence of RF signals, including such signals that may be frequency modulated. A frequency modulated continuous wave (FMCW) receiver is one type of receiver that is configured to identify and receive frequency modulated signals. An FMCW signal (RFFMCW) for example, may be transmitted by a radar system that is using frequency modulated signals for its operation, and these frequency modulated signals may occur across a wide frequency range. An FMCW receiver designed to detect such signals, therefore, must be capable of tracking the signal across the full range of the desired frequency band as the input signal is modulated.
Prior approaches to building a wide bandwidth FMCW receiver fall into four broad categories: analog mixer based receivers, fixed analog compressive filter receivers, analog convolver based receivers, and wideband digital receivers. Slight variations in architecture and implementation exist within each category, but the basic principles of operation and design remain the same.
FIG. 6 (Prior Art) is a block diagram for a prior FMCW architecture 600 that utilizes a mixer 602 and sweep control of the voltage control oscillator (VCO) 604 that provides the local oscillator (LO) mixing signal. As depicted, sweep control block 606 provides one or more control signals to an oscillator, such as VCO 604, to control the LO mixing signal output by the VCO 604 to the mixer 602. The mixer 602 mixes the incoming RF signals, such as an FMCW signal, with the mixing signal and outputs a resulting signal at a fixed intermediate frequency (IF). Next, a fixed narrow band filter 608 at an IF frequency filters the signal and provides the filtered signal to a narrow band analog-to-digital converter (ADC) 610. The ADC 610 outputs digital signals to the digital signal processor (DSP) block 612. The DSP block 612 then processes the digital signals to determine if a signal has been detected or confirmed. Problems with this prior approach include interference and spurs caused by the mixing process as well as signal-to-noise (SNR) problems.
This mixer-based architecture for a wide bandwidth FMCW receiver, therefore, uses a mixer to translate the wideband FMCW signal to an intermediate frequency (IF). At the intermediate frequency (IF), a fixed filter is used to separate the translated FMCW signal from background noise and interference, and in the case of digital processing, the fixed filter may be used to avoid aliasing. Following the fixed filter, energy detection can be performed. Various technologies for generating the mixing signal are possible, including the voltage controlled oscillator (VCO) shown in FIG. 6 (Prior Art), impulse excitation of a fixed compressive filter, direct digital synthesis, and acoustic charge transport techniques. Regardless of the technology used, however, the basic principles of this architecture are the same. The frequency of the input signal is matched by the sweep control driving the VCO. Detection can be performed at either IF or baseband using analog or digital techniques. Disadvantages of these techniques include spurs and phase noise introduced by the mixer. In addition, apriori information is required because the mixer tuning must approximately match the signal frequency at each instant in time in order to keep the tuned result within the fixed filter bandwidth.
Another prior architecture is a compressive filter architecture. Such an architecture may be based on using an analog matched filter whose impulse response is the time reversal of the desired signal to be received. Thus, when the desired signal is received, the output of the compressive filter is an impulse. The primary advantage of the compressive filter is that it forms the optimum receiver for the particular waveform of interest with maximum processing gain. Because the compressive filter is analog, far less power is required for wideband applications than an all digital approach. The compressive filter uses tapped delay lines (or the mathematical equivalent) to achieve the appropriate dispersion and may be implemented via various technologies, including surface acoustic wave (SAW) filters, superconductive electromagnetic material tapped delay lines, charge-coupled devices (CCDs), and optical/acousto-optic devices. Each of these technologies, however, has its disadvantages, especially when applied to typical electronic warfare (EW) applications. In general, CCDs do not have enough bandwidth for the typical EW applications. SAW filters can provide greater bandwidth than CCDs with very small form factor, but are limited in SNR (signal-to-noise ratio) and spurious dynamic range. Acousto-optic devices can also allow a small form factor receiver system, but they also have limited dynamic range. Superconductive tapped delay line techniques can provide extremely wide bandwidths, but typically suffer from very limited spurious dynamic range and the need for some form of cryogenic cooling. Another problem with the wider bandwidth devices is that they have limited programmability when used in a compressive receiver architecture since any given device is not able to produce impulse responses matching the time reversal of a wide range of frequency modulated signals.
Another prior architecture is a convolutional matched filter receiver. The optimum matched filter can also be achieved by a convolutional receiver that uses an analog convolver and a waveform synthesizer to convolve the desired signal with a time reversed replica. Thus, like the compressive receiver, the convolutional matched filter receiver produces an impulse output when the desired signal is received. This architecture is more flexible than the compressive receiver architecture, because the received signal structure is not fixed in the receiver implementation but is a synthesizable input to the convolver. Therefore, the convolutional matched filter receiver can provide optimum processing gain against a wide variety of input waveforms. The cost for this flexibility is the complexity of the RF waveform synthesizer. In addition, this architecture suffers from the device technology used in the analog convolver. SAW devices are typically used in convolutional matched filter receivers. These convolutional devices have limited dynamic range and bandwidth.
Still another prior architecture is a wideband digital receiver architecture. This architecture uses an IF bandpass filter with large bandwidth followed by a wideband high speed ADC. The matched filtering can then be performed digitally using a variety of techniques. Advantages of wideband digital receivers include flexibility, the ability to perform matched filtering against different signals simultaneously in the same receiver, and the ability to capture wide instantaneous bandwidth signals. Disadvantages include the limited dynamic range (decreased SNR and spurious-free dynamic range) of high speed ADCs compared to slower ADCs, as well as the higher power consumption required by the ADC and the processing of the high-speed digital data.
In short, these prior architectures fall short of providing an efficient solution for detecting wideband frequency-modulated, time-varying signals.